1. Field of the Invention
The present invention relates to an electrostatic recording apparatus of electrophotographic type, and more particularly to an electrostatic recording apparatus of reversal development type using a photoconductor drum having a photosensitive material or photoconductor sheet wound on a drum surface.
2. Description of the Related Art
A conventional technique concerning an electrostatic recording apparatus using a photoconductor drum having a photoconductor sheet wound on the surface is disclosed in Japanese Patent Unexamined Publication No.49-40737 (JP-A-49-40737). This conventional technique concerns an electrostatic recording apparatus using a photoconductor sheet wound drum, in which a photoconductor sheet is drawn out from a stock roll accommodated in the drum to the outside of the drum, via an opening formed in part of the drum, to be wound on the surface of the drum and is then taken in the drum through the opening again to be wound by a take-up roll. A metal portion of a drum cap of a normal development the copying machine is connected to a ground potential and has a surface on which high resistive film such as aluminum oxidized film is formed so that toner is not attached on the surface of the drum cap.
Further, a conventional technique concerning the electrostatic recording apparatus of this type is disclosed in JP-A-2-139583, proposed by inventors including one of the inventors of the present invention. In this conventional technique, a drum cap is utilized to measure a surface potential of a photoconductor on a photoconductor drum.
Generally, in an electrostatic recording apparatus of reversal development type such as a printer, in order to prevent toner from being attached on the surface of the cap in the development it is necessary to hold a surface potential of a cap to a sufficiently higher voltage than a developing bias voltage and to form a dielectric film on a surface of a metal portion of the cap so as to reduce an image force due to electric charges of toner particles. A method of forming the dielectric film having a volume resistivity of 10.sup.9 cm or less on the cap surface to reduce the image force by electric charges of toner particles has been proposed in JP-A-2-12571 by inventors including three of the inventors of the present invention.
In order to hold the surface potential of the cap to the sufficiently higher voltage than the developing bias voltage, there is a method of connecting a capacitor between the cap and the ground and charging the capacitor through a charging current from a charger to apply the charged-up potential to the cap. This method has merit in that a new power supply is not required. In this method, when the volume resistivity of a dielectric film formed on the cap surface is equal to 10.sup.9 .OMEGA.cm or less and a thickness of the film is in a range from 20 to 100 .mu.m, a resistance of the film is equal to 100 K.OMEGA. or less. In this case, when the resistance of the film is increased, the charging current is reduced and the charging speed is influenced. Further, a potential of the surface of the cap depends on the charging current, a charging time and a capacitance of the capacitor.
This method is now described with reference to FIGS. 12A and 12B. In FIG. 12A, numeral 1 denotes a drum, 2 a photoconductor sheet, 3-1 a stock roll, 3-2 a take-up roll, 5 a cap, 7-1 a capacitor, 7-2 a voltage control element, 10 a developer roll, 12 a developer, 13 a developing bias voltage supply, and 16 a charger. In an electrostatic recording apparatus, a photoconductor drum 4 is structured as shown in FIG. 12A. More particularly, the photoconductor sheet drawn from the stock roll 3-1 provided in the drum 1 is wound on a surface of the drum 1 made of aluminum or the like and is then wound on the take-up roll 3-2. The cap 5 made of aluminum or the like is disposed at an opening of the photoconductor drum 4. A parallel circuit 6 of the capacitor 7-1 and the voltage control element 7-2 such as a varistor is connected to the cap 5. Further, dielectric film 20-2 having a volume resistivity of 10.sup.7 .OMEGA.cm is formed on the surface of a metal portion 20-1 of the cap 5.
It is assumed that any electric charge is not stored in the capacitor 7-1 before start or during standstill of the electrostatic recording apparatus. When the electrostatic recording apparatus is started so that the drum begins to be rotated and the cap 5 passes under the charger 16 connected to high voltage supplies 17 and 18, a charging current through corona discharge flows to charge the capacitor 7-1. At this time, when a charging current flowing into the photoconductor drum 4 from the charger 16 is I.sub.d, a charging current of the capacitor 7-1 is I.sub.j, a charging time is T.sub.j, and a capacitance of the capacitor 7-1 is C.sub.j, a potential V.sub.c of the capacitor 7-1 (equal to a surface potential of the cap 5) is given by ##EQU1## When a moving speed of the drum 4 is v.sub.d, a width of the charger 16 is W.sub.c, and a width of the cap 5 is W.sub.d, the charging time T.sub.j and the charging current I.sub.j are expressed by EQU T.sub.j =W.sub.c /v.sub.d ( 2) EQU I.sub.j =I.sub.d .times.(W.sub.d /W.sub.c) (3)
That is, the voltage V.sub.c of the capacitor 7-1 is inversely proportional to the capacitance C.sub.j of the capacitor 7-1 and the moving speed v.sub.d of the drum and is proportional to the width W.sub.d of the cap 5 and the charging current I.sub.d. Accordingly, when the apparatus is operated at a high speed and the moving speed v.sub.d is increased, the charging time T.sub.j is made short and when it is considered to make the apparatus small, the width W.sub.d of the cap is also made narrow. However, it is not preferable for deterioration of the photoconductor and increased capacity of the power supply to increase the charging current I.sub.d excessively in order to charge the capacitor 7-1 rapidly.
On the other hand, as the cap 5 is apart from the charger 16, the charge stored in the capacitor 7-1 is leaked through the voltage control element 7-2 connected in parallel with the capacitor and the potential of the surface of the cap 5 is reduced. FIG. 12B shows a variation of the surface potential of the cap 5. When a time required for one rotation of the photoconductor drum 4 is T.sub.k, the surface potential V.sub.c of the cap 5 is increased gradually and in a step manner while repeating its increase and reduction. In FIG. 12B, T.sub.c is a time until the cap 5 reaches the developer 12 after the cap 5 comes under the charger 16, T.sub.d is a time until the cap comes under the charger 16 again after the cap comes out from the developer 12, and V.sub.G is a voltage of a grid of the charger 16. A potential V.sub.B of the cap 5 at the time when the cap passes under the developer 12 is gradually increased to V.sub.B1 in a first rotation, V.sub.B2 in a second rotation, V.sub.B3 in a third rotation and V.sub.B4 in a fourth rotation.
When the cap potential is varied as above, some problems occur as follows:
(1) Attachment of Toner on the Cap Surface
When a bias voltage of the developer 12 is V.sub.D, the potential or voltage V.sub.B of the cap is equal to or smaller than the bias voltage V.sub.D when the drum 4 is rotated three times. Accordingly, the cap 5 is developed and toner 21 is attached on the cap surface. Since the attached toner is removed by a cleaner, consumption of toner increases and scattered toner during the rotation of the drum contaminates paper and the interior of the apparatus.
(2) Electric Shock
There is a case where the photoconductor drum 4 is stopped suddenly due to a jam of paper or a failure of a printer to remove paper wound on the drum 4 or examine the drum. In this case, electric charges remain in the capacitor of the cap even after a time T.sub.D from stop of the drum 4 and the surface potential of the cap is V.sub.j. Accordingly, when the potential V.sub.j is high, an electric shock occurs due to contact with the cap 5.
(3) Influence of Step between the Cap and the Photosensitive Drum
Since there is a step between the surface of the cap 5 and the surface of the photoconductor drum 4, i.e., the cap is at a different outer radius with respect to the center of the drum than the surface of the drum there is a case where a flaw is produced on the surface of a belt pressed on the drum 4 by a roller when a belt transfer system is employed. Accordingly, there is a possibility that image quality is degraded and the belt is deteriorated. Further, there is a problem in view of retention of a uniform developing gap.